As a conventional sealant film for retort package in which retort sterilization is carried out at a high temperature of 120° C. to 135° C., a non-oriented film (hereinafter, also referred to as “CPP”) the main ingredient of which is propylene/ethylene block copolymer has been used. It has been used mainly as a packaging bag after laminating it with an ethylene terephthalate oriented film (hereinafter, also referred to as “PET”), a nylon oriented film (hereinafter, also referred to as “ON”) and an aluminum foil (hereinafter, also referred to as “Al foil”), thereby making a laminated material having a structure of PET/ON/Al foil/CPP, PET/Al foil/ON/CPP or PET/Al foil/CPP. In any of these retort bags, in order to preserve foods, which are contained substances, at room temperature for a long term, gas barrier properties (such as oxygen barrier property and water vapor barrier property) and a shading property together with a thermal resistance sufficient enough for bearing a high temperature at the time of sterilization have been required, and therefore, an Al foil has been used.
However, in a retort bag using an Al foil, there are defects that metal foreign matters entrapped into foods cannot be examined after charging the foods, that it cannot be cooked in a microwave oven, and that foods contained in the bag cannot be seen. Recently retort bags free from an Al foil have been desired, and barrier films, such as a film made by depositing silica onto PET or ON or a film made by coating an acrylic barrier agent, have been developed. Properties required for sealant films to be laminated with such metal-free barrier films include a content visibility, concretely, a transparency represented by haze value (a degree of scattering of transmitted light) and a see-through property. It is also important that a phenomenon in which a white trace remains at a portion having been bent or having received an impact (so-called, white crease by folding).
Conventional sealant films for retort package are laminated with opaque Al foil and therefore importance has not been attached to their transparency. They therefore were developed mainly aiming at improvement in low-temperature impact resistance, heat sealability and blocking resistance, etc. There therefore is no sealant film good in transparency and capable of being used satisfactorily as a usual retort packaging bag. For example, JP-A-2000-186159 and JP-A-2003-96251 disclose sealant films for retort food packaging excellent in low-temperature impact resistance, heat sealability and blocking resistance. The films in JP-A-2000-186159, however, have a haze value (: haze) of 45% or more, and the film of Example 1 of JP-A-2003-96251 has a haze value of 53%. Thus, both are poor in transparency. While a film good in transparency having a haze of 20% is disclosed in Comparative Example 2 of JP-A-2003-96251, the blocking resistance of the film is not good, and it cannot be used as a sealant film for retort package.
It could be advantageous to provide a polypropylene film which is good in transparency and see-through property and is free from the problem of white crease by folding, and which is suitable for use in ordinary retort applications as a packaging bag or a sealant for packaging bags, and has all of low-temperature impact resistance, heat sealability and blocking resistance, and a laminated material thereof.